Conventionally, an ultrasonic sensor, which is for detecting the distance from an object or existence of an object by transmitting an ultrasonic wave at a predetermined frequency and receiving a reflective wave reflected from an object, is known. In recent years, an ultrasonic sensor is used to detect an obstacle around a vehicle. A directivity characteristic of an ultrasonic sensor radially diffuses. Therefore, an ultrasonic sensor may erroneously detect a curbstone located on a road surface in a parking area, a road surface, and/or the like as an obstacle at a distance of several meters when the directivity characteristic of an ultrasonic sensor is not limited to a narrow range. For example, JP-A-2002-58091 discloses an ultrasonic sensor capable of further limiting a directivity characteristic. An ultrasonic sensor is required to have a wide directivity characteristic to detect an obstacle at a short distance of tens of centimeters. Therefore, an ultrasonic sensor having a wide directivity characteristic is used to detect an obstacle at a short distance.
However, the ultrasonic sensor of JP-A-2002-58091 having a narrow directivity characteristic cannot detect an obstacle in a wide detection area, such as a periphery of a vehicle, rather than an obstacle at a distance of several meters away. On the other hand, when an ultrasonic sensor has a wide directivity characteristic to detect an obstacle at a distance of several meters away, as described above, the ultrasonic sensor may erroneously detect a curbstone on a road surface in a parking area, a road surface, and/or the like as an obstacle. Therefore, the conventional structure has a problem that multiple ultrasonic sensor elements having different directivity characteristics are required respectively for different detection areas. It is conceived to switch among multiple directivity characteristics in one ultrasonic sensor element. However, it is difficult to obtain excellent directivity characteristics having different ranges in one ultrasonic sensor element.